1. Field of the Invention
The present invention relates to a pump drive motor control apparatus for controlling the rotational speed of a motor for driving a pump, and more particularly to a pump drive motor control apparatus which controls the rotational speed of a motor through on-off control performed such that supply of electricity to the motor is resumed on the basis of a result of comparison between a predetermined threshold and a voltage which the motor generates during a period in which supply of electricity to the motor is stopped.
2. Description of the Related Art
A conventional pump drive motor control apparatus of such a type is disclosed in, for example, Japanese kohyo (PCT) Patent Publication No. 2002-506406. The disclosed control apparatus is applied to a motor for driving a hydraulic pump which is used in an antilock brake system in order to pump brake fluid returned to a reservoir as a result of operation of the antilock brake system and to supply the pumped brake fluid to a hydraulic circuit of the antilock brake system. The control apparatus controls the rotational speed of the motor through on-off control performed such that supply of electricity to the motor is resumed when a voltage which the motor generates during a period in which supply of electricity to the motor is stopped (i.e., an induced electromotive force which the motor generates as a result of acting as a generator) becomes equal to or less than a predetermined threshold.
If the reservoir, to which brake fluid is returned as a result of operation of the antilock brake system, is filled with brake fluid, further return of brake fluid from the hydraulic circuit of the antilock brake system to the reservoir becomes impossible, resulting in failure of the antilock brake system to attain brake fluid pressure control (hereinafter, referred to as “ABS control”). Accordingly, brake fluid must be pumped out of the reservoir in order to prevent the brake fluid from filling the reservoir.
The time-average of flow rate at which the hydraulic pump pumps brake fluid out of the reservoir and discharges (hereinafter, may be simply referred to as “discharge flow rate”) increases with the time-average of rotational speed of a motor for driving the hydraulic pump (hereinafter, may be simply referred to as “rotational speed of the motor”). Further, the rotational speed of the motor increases as the predetermined threshold increases. However, in the disclosed apparatus, since the predetermined threshold is fixed, the rotational speed of the motor cannot be changed.
Moreover, the time average of quantity of brake fluid returned to the reservoir per unit time (hereinafter, may be simply referred to as “flow rate of brake fluid”) also varies in accordance with the operation conditions of the antilock brake system. Accordingly, in order to avoid failure of the ABS control in the disclosed apparatus, the rotational speed (constant value) of the motor is preferably set to a high level such that the pump can pump brake fluid from the reservoir at a flow rate corresponding to the expected maximum flow rate of brake fluid returned to the reservoir.
Meanwhile, a hydraulic pump and a motor as described above are required to reduce their operation sound (or noise) to the extent possible. The lower the rotational speed of the motor, the smaller the operation sound. In view of this, the rotational speed of the motor is strongly desired to be lowered to the extent possible.
Results of analysis of actual operation conditions of an antilock brake system revealed that in many cases, the flow rate of brake fluid returned to the reservoir is relatively low during a short period of time immediately after the start of ABS control. This will be described below.
In general, cases where ABS control is started can be classified into a case where an excessively abrupt braking operation is applied to the brake pedal when an emergency arises (hereinafter referred to as a “case where excessively abrupt braking is demanded”), and a case in which a driver his/herself controls the brake pedal depressing force (hereinafter simply referred to as “depressing force”) in such a manner that the actual deceleration of the vehicle body approaches the maximum attainable deceleration of the vehicle as determined by the friction coefficient of road surface (hereinafter referred to as a “case where depressing force control is performed”). Of these, the case where depressing force control is performed occurs more frequently.
In the case where the depressing force control is performed, since the driver controls the brake pedal depressing force, even when ABS control is started in response to locking of a wheel, the ABS control (temporarily) ends when the driver immediately decreases the brake pedal depressing force. In other words, in many cases, ABS control ends within a short period of time. Moreover, in such a short period of time, in many cases the difference between the pressure (i.e., master cylinder fluid pressure) generated within the antilock brake system in accordance with the brake pedal depressing force and the fluid pressure of the wheel cylinder on the locked wheel subjected to the ABS control is maintained relatively small. In general, the flow rate of brake fluid returned to the reservoir tends to decreases with the pressure difference. Accordingly, in the above-described short period of time, in many cases the flow rate of brake fluid returned to the reservoir becomes relatively low.
As is understood from the above, during a short period of time immediately after the start of ABS control, even when the rotational speed of the motor is set to a relatively low level, failure of the ABS control hardly occurs. Accordingly, during such a short period of time, it is advantageous to set the rotational speed of the motor to a low level with higher priority placed on reduction of operation sound over reliable avoidance of failure of ABS control. In other words, reducing the operation sound of the motor (pump) during a short period of time immediately after the start of motor speed control is preferred.